Central nervous system (CNS) disorders affect a wide range of the population with differing severity. For example, schizophrenia is a psychopathological disorder of unknown origin, which usually appears for the first time in early adulthood and is marked by characteristics, such as, psychotic symptoms, phasic progression and development, and deterioration in social behavior and professional capability. Characteristic psychotic symptoms include disorders of thought content (e.g., multiple, fragmentary, incoherent, implausible or simply delusional contents, or ideas of persecution) and of mentality (e.g., loss of association, flight of imagination, incoherence, or incomprehensibility), as well as disorders of perceptibility (e.g., hallucinations), emotions (e.g., superficial or inadequate emotions), self-perceptions, intentions, impulses, and inter-human relationships, and psychomotoric disorders (e.g. catatonia). Other symptoms are also associated with this disorder. See, e.g., Diagnostic and Statistical Manual of Mental Disorders, 4th Ed., American Psychiatric Association (1997) (DSM-IV™).
Schizophrenia can be classified into various subgroups. For example, the paranoid type is characterized by delusions and hallucinations and absence of thought disorder, disorganized behavior, and affective flattening. The disorganized type, also named hebephrenic schizophrenia, is characterized by the presence of both thought disorder and affective flattening. The catatonic type is characterized by prominent psychomotor disturbances, including symptoms of catatonic stupor and waxy flexibility. In the undifferentiated type, psychotic symptoms are present but the criteria for paranoid, disorganized, or catatonic types have not been met.
The symptoms of schizophrenia normally manifest themselves in three broad categories, i.e., positive, negative and cognitive symptoms. Positive symptoms are those that represent an excess of normal experiences, such as hallucinations, disorganized speech, and delusions. Negative symptoms are those where the patient suffers from a lack of normal experiences, such as anhedonia, lack of motivation, inability to experience pleasure, and lack of social interaction. The cognitive symptoms relate to cognitive impairment in schizophrenics, such as lack of sustained attention, impairment of memory, and deficits in decision making. The current anti-psychotics are somewhat effective in treating the positive symptoms but are less effective in treating the negative or cognitive symptoms. For instance, the current typical or atypical anti-psychotics do not address cognitive or negative symptoms of schizophrenia, and only treat the positive symptoms in approximately 40% of patients.
Cognitive impairments include a decline in cognitive functions or cognitive domains, e.g., working memory, attention and vigilance, verbal learning and memory, visual learning and memory, reasoning and problem solving, e.g., executive function, speed of processing and/or social cognition. In particular, cognitive impairment may indicate deficits in attention, disorganized thinking, slow thinking, difficulty in understanding, poor concentration, impairment of problem solving, poor memory, difficulties in expressing thoughts, difficulties in integrating thoughts, feelings and behavior, or difficulties in extinction of irrelevant thoughts.
Agitation is a well-recognized behavioral disorder with a range of symptoms, including hostility, extreme excitement, poor impulse control, tension, and uncooperativeness. Agitation is common in the elderly and often associated with dementia such as those caused by Alzheimer's disease, Parkinson's disease, and Huntington's disease, and by diseases that affect blood vessels, such as stroke or multi-infarct dementia, which is caused by multiple strokes in the brain. An estimated five percent of people aged 65 and older and up to 20 percent of those aged 80 and older are affected by dementia. Of these sufferers, nearly half exhibit behavioral disturbances, such as agitation, wandering, and violent outbursts. Agitated behaviors can also be manifested in cognitively intact elderly people and by those with psychiatric disorders other than dementia.
Dementia is characterized by several cognitive impairments including significant memory deficit and can stand alone, or be an underlying characteristic feature of a variety of diseases, including but not limited to, Alzheimer's disease, Parkinson's disease, Huntington's disease, and multiple sclerosis.
Thus, there remains a great need for effective treatments of various CNS disorders.
Cyclic nucleotide phosphodiesterases (PDEs) are a super family of enzymes encoded by twenty-one genes, and are subdivided into eleven known families based on structure and function. PDEs are modular enzymes having a catalytic domain in the C-terminal portion of the protein and regulatory elements in the N-terminal portion. PDEs hydrolyze the phosphodiester bond of cyclic nucleotides, e.g., cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP), converting them into the corresponding monophosphates. cAMP and cGMP function as intracellular second messengers regulating a wide range of intracellular processes. For instance, in neurons cAMP and cGMP activate cyclic-nucleotide-dependent kinases and the subsequent phosphorylation of proteins involved in acute regulation of synaptic transmission and in neuronal differentiation and survival. PDEs are therefore important regulators of a wide variety of physiological processes. PDEs are expressed differentially throughout the organism and cyclic nucleotide signaling is highly compartmentalized within individual cells. Thus, different PDE isozymes can serve distinct physiological functions. Compounds that can selectively inhibit distinct PDE families or isozymes may offer additional therapeutic benefits, fewer side effects, or both.
PDE-10 was first reported in 1999 (Soderling et al., Proc. Natl. Acad. Sci., 1999, 96, 7071-76; Loughney et al., Gene, 1999, 234, 109-17; Fujishige et al., J. Biol. Chem., 1999, 274, 18438-45). Homology screening revealed mouse PDE-10A as the first member of the PDE-10 family of enzymes. The human PDE-10 sequence is highly homologous to both the rat and mouse PDE-10 enzymes. The PDE-10 family of enzymes has a lower degree of sequence homology as compared to previously identified PDE families. PDE-10 can hydrolyze both cAMP (Km=0.26 μM) and cGMP (Km=7.2 μM), and has a five-fold greater Vmax for cGMP than for cAMP.
PDE-10A is primarily expressed in the brain, also found in testes. PDE-10A mRNA and protein are abundant in brain tissues, and are mainly detected at high levels in the medium spiny neurons (MSN) of the striatum, a distribution conserved across mammalian species. The striatal MSNs provide input to the basal ganglia circuit, affecting action selection and execution, and suppressing undesired responses to sensory stimuli. PDE-10A has become an emerging target for the development of new anti-psychotics. Inhibitors of PDE-10A have been shown to increase cAMP and cGMP levels in striatal tissue and have demonstrated efficacy against not only positive but also negative and cognitive symptoms in animal models of schizophrenia. PDE-10A is also useful in treating metabolic disorders, such as diabetes, obesity, and metabolic syndrome.
Citation of any references in this Section of the application is not to be construed as an admission that such reference is prior art to the present application.